Mean High Water
A project documenting the impacts of sea level rise & flooding across the South Carolina Lowcountry.
more
more
TABLE OF CONTENTS
USEFUL LINKS
Mean High Water
TABLE OF CONTENTS
USEFUL LINKS
A PRIMER ON FLOODING IN CHARLESTON
Charleston’s Flooding Challenge
Charleston has battled flooding since its founding in 1680. The city's low-lying topography, surrounded by the coastal waters of Charleston Harbor, creates natural vulnerability to inundation. While storms and high tides have caused flooding since before European settlement on Oyster Point, modern development decisions have significantly increased flood risks. Recent sea level rise trends have significantly increased the frequency of tidal flooding, presenting new challenges for citywide flood management.
Three Sources of Flooding
Charleston experiences flooding from three distinct sources: rainfall (stormwater), high tides, and storm surge. While the resulting floods may look similar, understanding their underlying causes and differences is crucial for effective flood management.
When rainfall rates exceed the city's drainage system capacity, water ponds in low-lying streets and areas. Stormwater runoff volumes can quickly overwhelm infrastructure, causing widespread flooding.
Stormwater flooding has always challenged Charleston, but urban development has worsened the problem. Construction of buildings and roads across the peninsula—particularly over former tidal creeks—has reduced natural drainage capacity. Many existing drainage systems were not designed to handle current rainfall volumes, leading to extensive flooding during heavy rain events.
Timing is difficult to forecast, but flood locations can be predicted based on topography, elevation, and drainage system conditions. Impact severity varies based on each storm's unique rainfall patterns.
When tides in the Atlantic Ocean, Charleston Harbor, and area rivers exceed certain thresholds, tidal waters back up through storm drains and overtop perimeter roadways, flooding low-lying areas.
Tide levels depend on lunar gravitational pull (and solar influence to a lesser extent), coastal topography, and meteorological conditions. While tides have been recorded since the 1920s, tidal flooding only recently became common—remarkably, over 64% of all major tidal floods have occurred since 2015.
Tidal floods can be forecast in advance, and flood locations are predictable based on elevation. However, the extent and severity remain difficult to predict precisely.
Tropical storms and hurricanes push ocean waters inland, often causing widespread regional flooding. Hurricane Hugo on September 22, 1989, produced Charleston's highest surge on record. The most recent significant surges occurred during Hurricane Idalia on August 30, 2023, and a Nor’easter on December 17, 2023.
Storm surge resembles tidal flooding but is typically more severe, featuring higher water levels and destructive waves.
Storm surge potential can be forecast in advance, but actual impacts are difficult to predict due to variables in storm path and the timing of approach relative to tidal cycles.
Key Takeaways
Each flooding source presents unique challenges and requires different management strategies. Understanding these distinctions helps residents, planners, and emergency managers prepare for and respond to Charleston's complex flooding environment.
Dive Deeper
Want to learn more about flooding in Charleston? I was part of a team that developed the Charleston Water Plan, a foundational strategy for managing flood risks and embracing water's place in the city's future.
The plan assesses flood risk in Charleston today and projects it out to 2050, accounting for 14 inches of sea level rise, rainfall-runoff, storm surge, and groundwater emergence. It establishes three key strategic themes:
Make Space for Water - Preserve and restore spaces where water is and where it wants to be.
Act now, adapt over Time - While adaptation benefits must justify their costs, those costs will precede the benefits. We must build adaptive capacity—the ability to adjust strategies as future conditions evolve.
Learn more about the Charleston Water Plan and its comprehensive approach to flood resilience: